1. Field of the Invention
This invention relates generally to injection molding. In particular, this invention relates to assemblies for controlling flow of melt through conduits between an injection unit and mold cavities.
2. Description of Related Art
Injection molding is a cyclic process wherein mold assemblies defining mold cavities are operable between “open” and “closed” conditions, the open condition permitting release of molded articles and the closed condition permitting filling of the cavities with material to be molded. Mold assemblies comprise mating mold components comprising a primary core component and primary cavity component, the primary core component and primary cavity component meeting along a so called “parting line” when the mold assembly is closed. The mating mold components define cavities establishing the shape and size of articles to be molded. A mold assembly within the press unit of a molding machine is illustrated in FIG. 1a. Filling of cavities is effected by forcing flowable material, e.g. molten metal, rubber or thermoplastic (known as “melt”), from an injection unit through conduits to the mold cavities. Once filled, the mold assembly is held closed while the molten material solidifies and is opened to allow removal of molded articles when the material has sufficiently solidified to retain the molded form without unacceptable distortion when the material is unsupported.
Improved productivity of injection molding is achieved by providing mold arrangements comprising plural mold assemblies having plural parting lines within a single injection molding machine mold clamping unit. In such arrangements mating mold components are carried on intermediate movable platens interposed between a stationary platen and a primary movable platen defining the machine clamping unit of the injection molding machine. Such arrangements are known as “stack molds” or “dual molds” and FIG. 1b illustrates such a mold arrangement in the press unit of a molding machine. Melt is conveyed from the injection unit to the mold cavities through conduits that must accommodate cyclic separation and joining of the mating mold components. Such conduits are sometimes referred to as sprue bars. It is known to provide means to retain melt within sprue bar segments while the mold assemblies are “open” (mating mold components separated). Such means may comprise valves to seal retained melt within the sprue bar segments or extending members to effectively depressurize retained melt relieving forces that otherwise would propel melt out of the sprue bar segments. Melt depressurization depends on achieving sufficient volumetric increase with mold opening to insure melt is retained within the sprue bar segments. Known devices rely on melt pressure to effect volumetric increase. In the event mold opening is completed before melt depressurization, melt can leak from the unsealed sprue bar segment. Independently actuated valves may be provided to seal the sprue bar segments independently mold opening. The added complexity of actuators for such valves increase the overall costs of mold assemblies so equipped. Hence it is desirable to provide for sealing sprue bar segments by devices which are passively actuated with opening of the associated mold assembly so as to overcome the disadvantages of the known devices.